In a common and well known prior art type of power wrenches the output torque is measured and determined via signals from a load indicator provided in the power transmission between the motor and the output shaft. This load indication is accomplished by a sensor provided between the stationary ring gear of a planetary reduction gearing and the power wrench housing and is in fact an indication of the reaction torque transferred via the ring gear to the housing. The torque delivered via the output shaft is then calculated from that measured reaction torque load on the ring gear. This gives in fact only an indirect value of the torque magnitude delivered via the output shaft, and since there inevitably are varying frictional conditions and tolerances of the components of the mechanical power transmission there will always be some deviations of the calculated output torque value from the delivered torque magnitude. Not least the mechanical wear of the angle drive in angle type power wrenches is a common source of error when determining the delivered output torque magnitude.
Another prior art method for determining the delivered torque of a power wrench comprises measurement of the torsional deflection of an inline torque transferring component of the power transmission during load application on the output shaft. This is a direct measurement of the transferred torque, but there are uncertainties depending on manufacturing tolerances, friction conditions, mechanical wear etc. which contribute to cause deviations and scattering of the determined torque magnitude delivered via the output shaft in comparison with the measured torque level. A problem is that there is no way to know whether a deviation is acceptable or not. This may lead to that an individual tightening operation may appear to be acceptable, where in fact the delivered target torque is far from the desired target torque due to an abnormal deviation.
To be able to get a reliable information of the delivered torque magnitude and, hence a high and approved quality of a tightening process via any of the above described prior art torque measuring methods there have been required repeated and costly individual power wrench calibration operations.